It has long been recognized that near-end cross talk (NEXT) between two signals transmitted at substantially the same high frequency in opposite directions over different pairs in the same communication cable, is not overcome merely by the use of differing pair twists for the transmitting pairs. Accordingly, in recent years, metallic screens have been used to promote further electrical isolation between one or more groups of conductors and other groups within the cable core. In a screened cable, many pairs of separate transmission paths for oppositely traveling signals of like frequency can be selected. The high energy amplified signal leaving a repeater, for example, is impeded by the screen from propagating into a pair carrying relatively weak, incoming signals.
Most screened cables currently in use, however, manifest a small but still significant coupling between certain pairs on opposite sides of the screen. A number of different mechanisms are responsible, including the transfer surface impedance of the screening material, and coupling around the screen.
Moreover, the addition of screens complicates somewhat the cable manufacturing process, especially for cables with petroleum jelly-filled cores. Partly due to the screen's presence, the jelly does not flow freely after introduction, resulting in occasional air pockets forming within the core.